Protocol for purification and identification of MHC class I immunopeptidome from cancer cell lines.

Major histocompatibility complexes (MHC) play a critical role in immunity by presenting peptides on the cell surface for T cell recognition. Identification of these peptides can be valuable to develop vaccines or immunotherapeutic strategies for infectious diseases and cancers. Mass spectrometry is the only tool available for unbiased identification of the immunopeptidome. Here, we describe a protocol for purification and identification of MHC class I peptides, including in-house purification of anti-MHC-antibody from hybridoma cells and the LC-MS/MS analysis of MHC-I bound peptides.

The Choice of HLA-Associated Peptide Enrichment and Purification Strategy Affects Peptide Yields and Creates a Bias in Detected Sequence Repertoire.

Understanding the most appropriate workflow for biochemical human leukocyte antigen (HLA)-associated peptide enrichment prior to ligand sequencing is essential to achieve optimal sensitivity in immunopeptidomics experiments. The use of different detergents for HLA solubilization as well as complementary workflows to separate HLA-bound peptides from HLA protein complex components after their immunoprecipitation including HPLC, C18 cartridge, and 5 kDa filter are described. It is observed that all solubilization approaches tested led to similar peptide ligand identification rates; however, a higher number of peptides are identified in samples lysed with CHAPS compared with other methods. The HPLC method is superior in terms of HLA-I peptide recovery compared with 5 kDa filter and C18 cartridge peptide purification methods. Most importantly, it is observed that both the choice of detergent and peptide purification strategy creates a significant bias for the identified peptide sequences, and that allele-specific peptide repertoires are affected depending on the workflow of choice. The results highlight the importance of employing a suitable strategy for HLA peptide enrichment and that the obtained peptide repertoires do not necessarily reflect the true distributions of peptide sequences in the sample.

Protein a Immunoadsorption May Hamper the Decision to Transplant Due to Interference With CDC Crossmatch Results.

Transplanting immunized patients requires immunological monitoring in the pretransplant phase to follow reduction of donor specific HLA antibodies (DSA) after Staphylococcus aureus protein A (SPA) immunoadsorption (IA) or therapeutic plasma exchange followed by IVIG and Rituximab administration. Pretreatment aims to significantly reduce DSA strength. The Tissue Typing Lab at Aarhus University Hospital performs immunological monitoring of approximately 150 kidney transplantation patients per year from two transplant centers. From 2012 to 2013, we experienced seven patients desensitized using SPA IA, initially presenting negative cytotoxic complement dependent (CDC) T-cell crossmatches but positive B and T cell flowcytometric crossmatch, who despite significant DSA reduction developed weakly positive CDC T-cell crossmatch shortly prior to transplantation. We hypothesised that leached SPA during IA could be the cause, as the complication was not observed in patients who received plasma exchanges. We found that the positive CDC was not donor specific and SPA column material incubated with control serum reproduced a positive CDC T-cell crossmatch. Finally, we detected leached SPA in one of the patient samples using a highly sensitive time-resolved fluorescent assay. In conclusion, the results emphasize the importance of carefully considering CDC crossmatch results subsequent to IA, before a planned transplantation is either postponed or cancelled. J. Clin. Apheresis 32:163-169, 2017. © 2016 Wiley Periodicals, Inc.

Enfermedad de Behçet pseudotumoral / Pseudotumoral Behçet's disease

La enfermedad de Behçet es una vasculitis caracterizada por úlceras bucales y genitales. La afectación neurológica o neuro-Behçet es una manifestación infrecuente, de predominio en el género masculino y que aparece de 2 a 4 años después de la primera manifestación clínica. El neuro-Behçet cursa ocasionalmente lesiones cerebrales pseudotumorales. Presentamos 2 casos de pacientes diagnosticados de neuro-Behçet tras la detección de lesiones cerebrales pseudotumorales y se realiza una revisión de la literatura (AU)

Behçet‘s disease is a systemic vasculitis characterized by the presence of oral and genital ulcers. Neurological involvement or neuro-Behçet is an uncommon manifestation. It manifestation has predominance in the male gender appearing 2 to 4 years after the first clinical manifestation. However, neuro-Behçet disease sometimes occurs with pseudotumoral brain lesions. Herein, we present the cases of two patients diagnosed with neuro-Behçet after detection of pseudotumoral brain lesions. A review of the literature is performed (AU)

HLA-DPA1*/DPB1* en asociación con leucemias linfoides agudas y leucemias mieloides crónicas en mestizos venezolanos / HLA-DPA1*/DPB1* in association with acute lymphoid leukemias and chronic myeloid leukemias patients in Venezuelan twins

Hoy conocemos algo más de la participación de la región HLA-DP en la patogénesis de las leucemias gracias a varios estudios demostrando la injerencia temprana de estas moléculas en la modulación de la respuesta inmunitaria frente a patógenos. Para la evaluación de alelos y haplotipos HLA-DPA1*/DPB1* (28 alelos del locus HLA-DPA1* y 123 alelos HLA-DPB1*) se utilizó PCR SSPTM Olerup (Genovision) en 48 pacientes con LLA, 48 con LMC y 48 controles mestizos venezolanos. Para la asociación HLA/leucemias se tomó el riesgo relativo (RR) > 3 como positivo y negativo RR < 3, con una p corregida < 0,05. Pacientes LLA confirmaron asociaciones positivas con el alelo DPA1*0105 y negativa con el alelo DPA1*010301-010302. Además, resultaron asociados positivamente DPA1*0106 y *0107; DPA1*020101-020106 se asoció negativamente con la LLA. DPA1*0105, *0108 y *0109 resultaron asociados negativamente con la LMC. Las frecuencias observadas de los alelos HLA-DPB1*01:01, 02:01, 03:01, 04:01 y 04:02 en los controles mestizos venezolanos ubicadas entre 7 y 16% fueron superiores a las de pacientes leucémicos. Se confinaron asociaciones negativas de los alelos DPB1*02:01 y DPB1*03:01 con las LLA. No se observaron asociaciones positivas con las LLA. DPB1*99:01 se asoció negativamente con las LLA. La región DPB1* sola no parece estar asociada a las leucemias en esta población venezolana. La fuerte asociación LMC con varios haplotipos DPA/DPB indica importantes diferencias entre las patogénesis de ambas enfermedades

More is now known of the involvement of HLA-DP region in the pathogenesis of the leukemias through several previousstudies showing interference of these molecules in modulating the immune response to pathogens. For evaluation of HLA alleles and haplotypes DPA1*/DPB1* (28 alleles HLA DPA1* and 123 of HLA- DPB1*) Olerup SSP ™PCR (Genovision) was used in 48 patients with ALL, 48 CML, and 48 Venezuelan twins as controls. For HLA/leukemias, a relative risk (RR) > 3 was considered to be a positive association and negative with an RR<3, with a p corrected P<.05. ALL patients confirmed positive associations with DPA1*0105 allele, and negative with DPA1*010301-010302. In addition, they were positively associated with DPA1*0106 and *0107, with DPA1*020101-020106 being negatively associated with ALL. DPA1*0105, *0108 and *0109 were negatively associated with CML. The observed frequencies of HLA-DPB1* 01:01, 02:01, 03:01, 04:01 and 4:02 alleles in Venezuelan, which twins were between 7 and 16%, were higher than those of leukemic patients. Negative associations of DPB1*2:01, *3:01 and LLA were confined. No positive associations were observed with ALL. Non-confirmed positive associations were observed between DPB1*99:01 and CML. Haplotypes HLA-DPA1*01:03-DPB1*4:01, *2:01, *99:01 were strongly positively associated with CML. DPA1*1:09-DPB1*2:01, *4:01 were negatively associated with the CML. DPA1*1:03-DPB1*4:02; DPA1*01:09-DPB1*2:01, *4:01 and DPA1*02:01-DPB1*04:02 were negatively associated with ALL. The DPB1* single region does not appear to be associated with leukemia in the Venezuelan population. The strong association with several haplotypes DPA*1/DPB1* and LMC suggests massive differences between the pathogenesis of both diseases

Next-generation HLA sequencing using the 454 GS FLX system.

Next-generation sequencing (NGS) of HLA class I and II loci (HLA-A, HLA-B, HLA-C, DRB1, DRB3, DRB4, DRB5, DQA1, DQB1, DPB1) is described here in detail using the 454 Life Sciences GS FLX System and Titanium chemistry. An overview of the protocol with our experience on sequence performance efficiencies, read depth and ambiguity analyses using the GS FLX System are also presented. A total of 14 HLA primer pairs with multiplex identifiers (MIDs) are used in clonal, amplicon-based pyrosequencing of up to 44 samples per plate using the GS FLX. Genotype assignment and ambiguity reduction -analysis is performed using Conexio Assign ATF 454 software. Clonal NGS gives a significant reduction in genotyping ambiguity during analysis of the highly complex HLA system.

Diabetes tipo 1 y autoinmunidad / Diabetes type 1 and autoimmunity

La diabetes mellitus de tipo 1 es una enfermedad crónica autoinmune caracterizada por la destrucción selectiva de las células beta de los islotes de Langerhans. Este proceso, probablmente causado por uno o más factores ambientales, tiene lugar en individuos genéticamente susceptible portadores de un HLA determinado, y puede ocurrir durante meses o años antes de que la enfermedad se ponga de manifiesto. La autoinmunidad está en relación con células T autorreactivas, es decir, células que desencadenan una reacción autoinmune contra antígenos propios. La fase preclínica de la diabetes puede ser detectada por la presencia de anticuerpos anticélulas beta (ICA), anticuerpos antiinsulina (IAA), y anticuerpos frente a la decarboxilasa del ácido glutámico (GAD). Dadas sus características específicas inmunológicas, genéticas y metabóicas, el riesgo para desarrollar una diabetes de tipo 1ª es bastante predecible. Aunque hoy día aún no disponemos de una estrategia terapétuica adecuada, es importante identificar a estas personas puesto que podrían beneficiarse de actuaciones terapéuticas para prevenir la enfermedad. La diabetes mellitus tipo 1 se asocia con frecuencia a otras enfermedades de etiología o patogenia autoinmune: enfermedad tiroidea autoinmune, en enfermedad celíaca y, en menor grado, a la enfermedad gástrica autoinmune y a la enfermedad de Addison. La determinación de los autoanticuerpos órgano-específicos proporcionan una manera sencilla de cribado de la autoinmunidad en una población susceptible, como es la de los enfermos diabéticos tipo 1, con la posibilidad de prevenir morbilidad y mortalidad (AU)

Type 1 diabetes mellitus (DM1) is a chronic immune mediated disease characterized by the selective destruction of insulin-producing beta cells in the islets of Langerhans. This process, probably caused by one or more environmental factors, takes place in HLA-genetically susceptible individuals and may be present for months or years before the disease manifests itself. Diabetes autoimmunity is related to autoreactive T cells, this is, T cells that trigger an immune reaction against self antigens. The preclinical period of the DM1 can be detected by the presence of islet cell antibodies (ICA), insulin autoantibodies (IAA), and antibodies to glutamic acid decarboxylase (GAD). Because of its specific immunological, genetic and metabolic character, the risk of developing DM1 is very predictable. Even if we still do not have an adequate therapeutic response/strategy it is important to identify those susceptible subjects in order to help them prevent the disease. DM1 is commonly related to other autoimmune diseases: thyroiditis and celiac disease and much less to pernicious anemia and Addison´s disease. The detection of organ specific autoantibodies in an easy way to determine autoimmunity among a susceptible population such as DM1 patients, and thus helping to prevent morbidity and mortality (AU)

HPLC and MS analysis for the identification and characterisation of peptides presented in the context of the non-classical human leukocytes antigen (HLA) class I molecule HLA-E.

In addition to a variety of other techniques used in T-cell epitope identification, mass spectrometery coupled to liquid chromatography have now become an important and sensitive tool in separation, detection, and sequence analysis of highly complex natural major histocompatibility complex (MHC) ligand mixtures. In this article, we present current strategies for the identification of MHC eluted peptides using high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) with a particular recall to those presented in the context of the non classical human leukocytes antigen (HLA) class I molecule HLA-E. In addition we also discuss the advantages and disadvantages of the methods available in the literature to concentrate and fractionate the peptides prior to analysis by mass spectrometry. An application of our method for isolation and characterization of peptides presented in the context of HLA-E is finally reported.

Qué, cómo, cuándo y hasta cuándo tratar una artritis indiferenciada / Treating undifferentiated arthritis. What, when, how and how long?

Con la instauración de las clínicas de artritis de reciente comienzo se valoran pacientes cada vez más precozmente en el curso de su enfermedad, por lo que un porcentaje importante de éstos aún no puede clasificarse dentro de un diagnóstico específico según los criterios de clasificación del American College of Rheumatology (ACR). En estos pacientes con artritis indiferenciada (AI) incluso más importante que establecer un diagnóstico es distinguir entre aquéllos que desarrollarán una artritis persistente y/o erosiva y, por tanto, serán candidatos a recibir tratamiento precoz con fármacos modificadores de la enfermedad (FAME) aunque no cumplan los criterios de clasificación del ACR, y los que tendrán un cuadro autolimitado. Los marcadores serológicos junto con las características clínicas en el momento de la presentación, integrados en modelos clínicos predictivos, son los instrumentos con los que actualmente cuenta el clínico para poder identificar a estos pacientes. Algunos estudios han demostrado las ventajas del tratamiento precoz en la AI (AU)

With the establishment of early arthritis clinics, patients can now be increasingly attended early in the course of their disease. This means that a significant proportion of these patients cannot be classified into a specific diagnosis using the traditional American College of Rheumatology (ACR) classification criteria. In these patients with undifferentiated arthritis (UA), even more important than assigning a diagnosis is the need to distinguish between patients who will develop a persistent and/or erosive disease and will be candidates for prompt treatment with disease-modifying antirheumatic drugs (DMARD), and patients in whom the disease is self-limiting. Serologic markers in combination with clinical features at presentation, integrated into predictive models, are the tools currently available to the clinician for identifying these patients. Several studies have demonstrated the advantages of early treatment in UA (AU)

Clinical characteristics of patients with hepatitis C virus-related chronic liver disease seropositive for anticentromere antibody.

The association between anticentromere antibody (ACA) and hepatitis C virus (HCV) infection remains unclear. We subjected eight patients with HCV-related chronic liver disease (CLD) seropositive for ACA to a battery of clinical and laboratory tests. The patient cohort was dominated by females, and four of the eight (50%) patients had a concomitant autoimmune disease. All of the patients had high titers of ACA (>or=1:320). The histological activity index scores in chronic hepatitis C (CH-C) patients with ACA were significantly higher than those in CH-C patients without antinuclear antibody (ANA) (12.8 +/- 1.8 vs. 8.3 +/- 4.5, P = 0.0372). The frequency of human leukocyte antigen (HLA) DR-8 in patients with HCV-related CLD seropositive for ACA was significantly higher than that in patients with CH-C seronegative for ANA (71 vs. 18%, P = 0.0108). These findings suggest that ACA is induced by chronic HCV infection in association with HLA DR-8, and that CH-C patients with ACA exhibit more severe hepatic fibrosis and inflammation than CH-C patients without ANA.

Mimetic human leukocyte antigen epitopes: shown by monoclonal antibodies and extra antibodies produced on transplantation.

BACKGROUND: Transplant patients often produce human leukocyte antigen (HLA) antibodies against their donors and produce more specificities than can be accounted for by HLA antigen mismatches. We theorize that the presence of extra, otherwise unexplainable specificities could be accounted for if antibodies reacted to more than one epitope (primary and mimetic) on distinct HLA molecules. The theory states that mimetic epitopes consist of the same three amino acids that comprise the primary, sterically placed approximately the same distance apart as are the corresponding amino acids of the primary. METHODS: A mimetic epitope table containing all primary epitopes and corresponding mimetic epitopes was built. Then, the HLA specificities of monoclonal and single patient antibodies were determined. These specificities that could not be defined by unique position or amino acid epitopes alone were then used to query the mimetic epitope table. RESULTS: A single antibody from a transplant patient and three monoclonal antibodies produced reactions that can best be explained as the result of one antibody reacting to the same amino acids at two distinct sites on the molecule. Those position and amino acid combinations (pos/aa) are the primary and mimetic epitopes. Using computerized methods of searching, mimetic epitopes were found in five additional kidney transplant patients who produced nondonor-specific antibodies in addition to donor-specific antibodies. CONCLUSIONS: Epitopes on the HLA molecule that mimic the primary epitope have been found. We suggest that these mimetic epitopes explain the additional antibodies often found on HLA immunization resulting from allograft rejections, pregnancies, and transfusions.

Long-term immunity against actual poxviral HLA ligands as identified by differential stable isotope labeling.

Viral peptides are presented by HLA class I on infected cells to activate CD8(+) T cells. Several immunogenic peptides have been identified indirectly by epitope prediction and screening of T cell responses to poxviral vectors, including modified vaccinia virus Ankara (MVA) currently being tested as recombinant or smallpox vaccines. However, for the development of optimal vaccination and immunomonitoring strategies, it is essential to characterize the actual viral HLA ligand repertoire of infected cells. We used an innovative approach to identify naturally processed MVA HLA ligands by differential HPLC-coupled mass spectrometry. We describe 12 viral peptides presented by HLA-A*0201 and 3 by HLA-B*0702. All HLA-A*0201 ligands participated in the memory response of MVA-immune donors, and several were immunogenic in Dryvax vaccinees. Eight epitopes were novel. Viral HLA ligand presentation and viral protein abundance did not correlate. All ligands were expressed early during the viral life cycle, and a pool of three of these mediated stronger protection against a lethal challenge in mice as compared with late epitopes. This highlights the reliability of the comparative mass spectrometry-based technique to identify relevant viral CD8(+) T cell epitopes for optimizing the monitoring of protective immune responses and the development of effective peptide-based vaccines.

Soluble HLA-G is absent from human embryo cultures: a reassessment of sHLA-G detection methods.

In recent years, the number of patients receiving in vitro fertilization (IVF) has been increasing, though the rate of successful implantations has remained at 10-20%. A major goal of this procedure is to afford the ability to select embryos with the most potential for implantation and development. Previous studies claimed to have detected soluble HLA-G (sHLA-G) protein in culture supernatant from 2 to 3-day embryos using ELISA methods, and concluded that sHLA-G protein levels were associated with successful implantation. This result, if substantiated could provide an important tool for IVF. In this study, we have re-examined these experiments by attempting to detect sHLA-G in the medium from 2 to 3-day embryos (84 samples) and 4 to 6-day embryos (25 samples) in which a part of blastocyst has started to differentiate into trophoblasts. Using a highly specific and sensitive ELISA, no sHLA-G protein was detectable in any sample, despite the fact that 27 of the 109 samples were from successfully implanted embryos. These results indicate that 2-6-day embryos do not secrete sHLA-G detectable by ELISA, and therefore that sHLA-G in culture medium is not a useful for successful implantation at this stage of development.

HLA-specific B cells: I. A method for their detection, quantification, and isolation using HLA tetramers.

BACKGROUND: The development of highly sensitive and specific assays for detecting and characterizing HLA-specific antibodies has contributed to an appreciation of the extensive involvement of those antibodies in graft injury and dysfunction. However, understanding the regulatory processes of the humoral response to transplantation and the mechanisms underlying therapeutic agents and protocols for preventing and treating sensitization requires a way to study HLA-specific B cells. METHODS: Lymphocyte preparations enriched for B cells were stained with one or more of three different HLA tetramers. Tetramer-positive (tet+) B cells were enumerated and evaluated for an association of their frequencies with known sensitization. In some cases, tet+ B cells were isolated and placed in culture with supplements known to activate B cells in a nonspecific fashion. RESULTS: For all tetramers used, the frequencies of tet+ B cells were significantly higher (4.1%-5.5%) among sensitized patients than among nonsensitized patients (1.6%-3.2%, P<0.001). Binding of the tetramers occurred by the surface immunoglobulin antigen receptor with little or no binding to antibody captured in the Fc receptor. Cultured tet+ B cells produced antibodies specific for epitopes of the tetramer antigen. There appeared to be a certain amount of crossreactivity in the binding of tetramers. The frequency of CD27+ cells among tet+ B cells was higher, on average (34.4%-38.8%) than among all B cells (26.2%) whereas the frequencies of CD38 were comparable in the two groups. CONCLUSIONS: Staining with HLA tetramers provides a means for identifying, quantifying, and isolating HLA-specific B cells.

Reversible HLA multimers (Streptamers) for the isolation of human cytotoxic T lymphocytes functionally active against tumor- and virus-derived antigens.

The development of MHC/peptide multimers has facilitated the visualization and purification of antigen-specific T cells. However, the persistence of multimers leads to prolonged T cell receptor signaling and subsequently to altered T-cell function. We have recently developed a new type of MHC/peptide multimers, which can be dissociated from the T cell. Herein, we have generated and tested for the first time reversible HLA/peptide multimers, termed Streptamers, for the isolation of human T cells. The Streptamer technique demonstrates the specificity and sensitivity of conventional HLA/peptide tetramers with regards to the sorting of human T lymphocytes. This is shown for T cells directed against immunogenic peptides derived from viral and tumor-associated antigens. We show that antigen-specific cytotoxic T cells remain functionally active following Streptamer dissociation, whereas lytic function and proliferation of the T cells is impaired in the presence of conventional tetramers. These novel HLA/peptide Streptamer reagents allow the isolation of antigen-specific T cells with preserved function and, therefore, facilitate the development of adoptive T cell transfer regimens for the treatment of patients with cancer or infectious diseases.

Targeted identification of infection-related HLA class I-presented epitopes by stable isotope tagging of epitopes (SITE).

Identification of peptides presented in human leukocyte antigen (HLA) class I molecules after viral infection is of strategic importance for immunology and vaccine development. A powerful strategy aimed at the rapid, unambiguous identification of naturally processed HLA class I-associated peptides, which are induced by viral infection, is presented here. The methodology, stable isotope tagging of epitopes (SITE), is based on metabolic labeling of endogenously synthesized proteins during infection. This is accomplished by culturing virus-infected cells with stable isotope-labeled amino acids that are expected to be anchor residues for the human leukocyte antigen allele of interest. Subsequently, these cells are mixed with an equal number of noninfected cells, which are cultured in normal medium. Finally, peptides are acid-eluted from immunoprecipitated HLA molecules and subjected to two-dimensional nanoscale liquid chromatography-mass spectrometry analysis. Virus-induced peptides are identified through computer-assisted detection of characteristic, binomially distributed ratios of labeled and unlabeled molecules.

The production, purification and crystallization of a soluble form of the nonclassical MHC HLA-G: the essential role of cobalt.

HLA-G is a nonclassical class I major histocompatibility complex (MHC) molecule that is primarily expressed at the foetal-maternal interface. Although the role of HLA-G has not been fully elucidated, current evidence suggests it protects the foetus from the maternal immune response. In this report, HLA-G (44 kDa) is characterized by expression in Escherichia coli. The inclusion bodies were refolded in complex with a peptide derived from histone H2A (RIIPRHLQL), purified and subsequently crystallized. Correct refolding was determined using two conformation-dependent antibodies. Cobalt ions were shown to be an essential ingredient for obtaining diffraction-quality crystals. The crystals, which diffracted to 1.9 A resolution, belonged to space group P3(2)2(1), with unit-cell parameters a = b = 77.15, c = 151.72 A.

Preparation and crystallization of the disulfide-linked HLA-G dimer.

HLA-G is a non-classical MHC class I, which binds to inhibitory receptors, such as Leukocyte Ig-like receptors, to induce a wide range of tolerogenic immunological effects. HLA-G can be expressed as a disulfide-liked dimer both in solution and at the cell surface. However, the three-dimensional structure of the HLA-G dimer is unknown. Here, we report the crystallization of the disulfide-linked dimer form of HLA-G by adding dithiothreitol (DTT), enabling a 3.2-A data set to be collected. We also show that DTT promotes disulfide bond exchange of refolded HLA-G, whose free cysteine was protected, thus facilitating its dimerization. This technique could also be applied for disulfide-mediated dimer/multimer formation of refolded proteins harbouring free cysteines.

Blastocyst MHC, a putative murine homologue of HLA-G, protects TAP-deficient tumor cells from natural killer cell-mediated rejection in vivo.

Blastocyst MHC is a recently identified mouse MHC class Ib gene, which is selectively expressed in blastocyst and placenta, and may be the mouse homolog of HLA-G gene the products of which have been implicated in protection of fetal trophoblasts from maternal NK cells and evasion of some tumor cells from NK cell attack. In this study, we identified two blastocyst MHC gene transcripts encoding a full-length alpha-chain (bc1) and an alternatively spliced form lacking the alpha2 domain (bc2), which may be homologous to HLA-G1 and HLA-G2, respectively. Both placenta and a teratocarcinoma cell line predominantly expressed the bc2 transcript. When these cDNAs were expressed in TAP-deficient RMA-S or TAP-sufficient RMA cells, only bc1 protein was expressed on the surface of RMA cells, but both bc1 and bc2 proteins were retained in the cytoplasm of RMA-S cells. Significantly, the RMA-S cells expressing either bc1 or bc2 were protected from lysis by NK cells in vitro. This protection was at least partly mediated by up-regulation of Qa-1(b) expression on the surface of RMA-S cells, which engaged the CD94/NKG2A inhibitory receptor on NK cells. More importantly, the bc1- or bc2-expressing RMA-S cells were significantly protected from NK cell-mediated rejection in vivo. These results suggested a role for blastocyst MHC in protecting TAP-deficient trophoblasts and tumor cells from NK cell attack in vivo.